1. Field of the Invention
The present invention relates to an image forming apparatus such as a thermal printer that uses a thermal head and to a method for correcting the energizing time data for heating elements in the thermal printer.
2. Description of the Prior Art
There have recently been developed image forming apparatuses adapted to print printing data (YMC data) obtained by converting and expanding image data (RGB data) from a digital still camera (digital camera). Since YMC data is composed of tone data, it is suitable to use an image forming apparatus having high tone reproducibility. Hence, there have been developed thermal transfer type image forming apparatuses (e.g. thermal printers) that are considered to have higher tone reproducibility relative to inkjet type image forming apparatuses.
In general, inkjet type image forming apparatuses have only two alternative values whether or not to fire an ink droplet onto one pixel. Therefore, in such image forming apparatuses, small ink droplets are to be placed on paper to try to ensure resolution and reproducibility through an error diffusion method, etc. On the contrary, in thermal transfer type image forming apparatuses (e.g. thermal printers), controlling the heat quantity to be applied in a recording operation allows the number of tones for one pixel to be increased.
Thermal printers commonly comprise a thermal head (line head) with more than thousands of heating elements (dot heating elements) arranged thereon in line (in the main scanning direction). Then, the thermal head is adapted to heat an ink film to transfer colorant (ink) of the ink film onto receiver paper (sheet) that is carried. Therefore, the amount of ink to be transferred to a sheet fluctuates in accordance with the heat quantity of the thermal head, and the fluctuation is utilized to form an image with rich tones on the sheet.
Accordingly, controlling the temperature (heat quantity) of the thermal head becomes an important factor to improve tone reproducibility for printing data. To be more concrete, it becomes important to control the energizing time for the thermal head.
For example, the thermal printer described in Japanese Patent Laid-Open Publication No. Hei 7-52436 (refer to FIGS. 2 and 3, etc.) performs a corrective operation in consideration of “the effect of adjacent heat (adjacent heat effect)” from the two left and right dots (four dots in total: Bn, Cn, En, and Fn) of a target heating element Dn (one dot) and “the effect of heat history (heat history effect)” from the five dots (Bn-1, Cn-1, Dn-1, En-1, and Fn-1) in the first preceding line as shown in FIG. 15, and then controls the energizing time for the thermal head.
Such a corrective operation (correction) is performed to prevent a situation where the adjacent heat effect and the heat history effect cause the density of the dot corresponding to the heating element Dn to be increased when the heating element Dn is simply energized for normal time (non-corrective normal time) based on YMC data.
That is, in consideration of the adjacent heat effect and the heat history effect, the printer is controlled so as to give a desired density by making the energizing time for the heating element Dn shorter than the normal time (non-corrective normal time) and thereby subtracting remaining heat energy from the original heat energy that is to be given based on YMC data.
To make a detailed description with reference to FIG. 16, the printer is controlled so as to subtract remaining heat energy (corresponding to the adjacent heat effect and the heat history effect) from the energy applied for the normal time (non-corrective normal time).
Also, in Japanese Patent Laid-Open Publication No. Hei 6-255141 (refer to claim 1, etc.), in order to subtract remaining heat energy as mentioned above, there is provided a temperature sensor for measuring the temperature of the thermal head and a cooling fan is adapted to send air toward the thermal head based on the measurement result of the temperature sensor.
However, the thermal printer described in Japanese Patent Laid-Open Publication No. Hei 7-52436 takes adjacent heat effect into much consideration in the last phase of energizing time (in the end of energizing time) for one dot as shown in FIG. 16. That is, the energizing time in the last phase of energizing time for one dot is shortened to prevent the temperature of the thermal head from being increased excessively.
Therefore, if remaining heat energy (corresponding especially to the adjacent heat effect) may be subtracted excessively from the energy applied for the normal time, the temperature of the thermal head is to be reduced too much when printing the next line (refer to FIG. 15), resulting in a possibility of reducing the density of the next line.
Also, in the case where a cooling fan sends air toward the thermal head to subtract remaining heat energy stored in the thermal head as in the thermal printer described in Japanese Patent Laid-Open Publication No. Hei 6-255141, there is a possibility that the thermal head may be cooled excessively. In this case, the temperature of the thermal head is to be reduced too much when printing the next line (refer to FIG. 15), resulting in a possibility of reducing the density of the next line, as is the case with the foregoing example.